Among the following reactions of hydrogen with halogens, the one that requires a catalyst is
(a) H2

Q: Why is Fluorine so reactive even though it has a stable covalent bond?

Fluorine is exceptionally reactive because its small atomic size forces the lone pairs of electrons on the two bonded F atoms very close together. This creates strong interelectronic repulsion, making the F-F bond surprisingly weak and easy to break.

Q: Do we need a catalyst for the reaction of hydrogen and chlorine?

No, a catalyst is not required. The reaction between H2 and Cl2 is photochemical, meaning it is initiated by sunlight or UV light, which provides the energy to break the Cl-Cl bond and start a rapid chain reaction.

Q: What kind of catalyst is typically used for the iodine reaction?

Finely divided Platinum (Pt) is often used as a catalyst for the reaction between hydrogen and iodine, along with heating.

Q: Why does the reactivity decrease down the halogen group?

As you go down the group, the atomic size increases and electronegativity decreases. This means the halogens become less effective at attracting electrons to form strong bonds with hydrogen. The resulting H-X bonds become progressively weaker from HF to HI.

Q: Is the reaction of hydrogen and iodine reversible?

Yes, it is highly reversible. Because the H-I bond is relatively weak, hydrogen iodide can easily decompose back into hydrogen and iodine gases upon heating.

Question

Among the following reactions of hydrogen with halogens, the one that requires a catalyst is

Options:

(A) $	ext{H}_2 + 	ext{Cl}_2 \longrightarrow 2	ext{HCl}$

(B) $	ext{H}_2 + 	ext{I}_2 \longrightarrow 2	ext{HI}$

(C) $	ext{H}_2 + 	ext{F}_2 \longrightarrow 2	ext{HF}$

(D) $	ext{H}_2 + 	ext{Br}_2 \longrightarrow 2	ext{HBr}$

Sigma Percentile · Accepted Answer

### The Halogen Family: A Spectrum of Reactivity

When we look at Group 17 of the periodic table, we encounter the halogens: Fluorine, Chlorine, Bromine, and Iodine. These elements are known for their high electronegativity and strong desire to grab an electron to complete their octet. However, their eagerness to react is not uniform. It varies dramatically as we travel down the group.

Imagine introducing hydrogen gas to each of these halogens. The responses you get range from violently explosive to incredibly sluggish. This question tests our understanding of this exact behavioral spectrum.

### The Explosive and the Sluggish

Let's start at the top of the group with Fluorine ($	ext{F}_2$). Fluorine is the most reactive non-metal in the periodic table. If you mix hydrogen and fluorine, they will react explosively even in the dark!

$$	ext{H}_2 + 	ext{F}_2 \longrightarrow 2	ext{HF}$$

Moving down to Chlorine ($	ext{Cl}_2$), the reaction is still vigorous but requires a little push, typically in the form of sunlight (photochemical reaction). Bromine ($	ext{Br}_2$) is even less enthusiastic and requires heating to react with hydrogen.

Finally, we reach Iodine ($	ext{I}_2$). Iodine is a large, heavy atom. Its hold on its outermost electrons is relatively weak, and its electronegativity is the lowest among the common halogens.

### Why Does Reactivity Drop?

The dramatic drop in reactivity down the group is governed by two main factors:

**1. Bond Dissociation Energy:** Fluorine has an unusually low $	ext{F}-	ext{F}$ bond dissociation energy because its small size causes intense repulsion between the lone pairs of electrons on the two bonded atoms. This makes it very easy to break the $	ext{F}_2$ molecule apart to start a reaction. Iodine, being large, doesn't have this intense lone-pair repulsion, making it less eager to break apart.

**2. Product Stability:** The $	ext{H}-	ext{F}$ bond formed is incredibly strong, releasing a massive amount of energy (highly exothermic). In contrast, the $	ext{H}-	ext{I}$ bond is much longer and weaker. The overall energy payoff for iodine reacting with hydrogen is quite low.

### The Verdict

Because of its low reactivity, mixing hydrogen and iodine gases at room temperature will result in almost nothing happening. To force them to react at a measurable rate, we must not only heat them but also introduce a **catalyst** (often Platinum, $	ext{Pt}$).

$$	ext{H}_2 + 	ext{I}_2 \stackrel{	ext{Pt, } \Delta}{ightleftharpoons} 2	ext{HI}$$

The catalyst provides an alternative reaction pathway with a lower activation energy, allowing the sluggish iodine molecules to finally bond with hydrogen. Furthermore, because the $	ext{H}-	ext{I}$ bond is relatively weak, this reaction is notably reversible.

Therefore, the correct answer is the reaction between hydrogen and iodine.

Animated Solution for Chemistry - s and p-Block Elements: Among the following reactions of hydrogen with halogens, the one that requires a catalyst is

Select Answer:

Visualized Solution

$Analyzing the Reactions$

$Reactivity Trend of Halogens$

$Reasons for the Trend$

$The Need for a Catalyst$

$Conclusion$

The Sigma Insight: Group 17 Elements

The Halogen Family

The Explosive and the Sluggish

Why Does Reactivity Drop?

The Verdict

Similar Questions

Which among the following is the most reactive?

The correct order of the thermal stability of hydrogen halides (H—X) is

The reaction of $HClO_{3}$ with $HCl$ gives a paramagnetic gas, which upon reaction with $O_{3}$ produces

Choose the incorrect statement.

The correct order of bond dissociation enthalpy of halogens is

The absolute value of the electron gain enthalpy of halogens satisfies

Identify the incorrect statement among the following

The products obtained when chlorine gas reacts with cold and dilute aqueous NaOH are

The number of halogen (s) forming halic (V) acid is

In the following reactions, products $(A)$ and $(B)$ , respectively, are $NaOH + Cl_{2} \to (A) + side products$ (hot and conc.) $Ca(OH)_{2} + Cl_{2} \to (B) + side products$ (dry)